Generally, asthma is a chronic respiratory disease, and an asthmatic attack narrowing the respiratory tract is a respiratory disease capable of causing a patient to die (A. Greening, “General principles”, In: Manual of asthma management, pp. 331-337, Ed. by P. M. O'byne, N. C. Thomson, W. B. Saunders, New York, 2001). Therefore, an asthma patient must monitor himself or herself, typically performs a forced expiration using a conveniently portable Peak Expiratory Flow Meter (PEFM), and measures Peak Expiratory Flow (PEF) at that time. Typically, the asthma patient recognizes his or her status on the basis of a peak expiratory flow value measured by the PEFM, and then determines whether to go to hospital.
The above-described conventional PEFM is operated on the basis of the principles of  FIG. 1, in which one end of a spring 11 having elasticity is fixed to a respiration tube 10 {circle around (s)} within the respiration tube 10, and the other end thereof is connected to a movable plate 12 movably arranged in the respiration tube 10. Further, an indicator 13 extending through the wall of the expiration tube 10 is arranged to be in contact with the movable plate 12. In FIG. 1, when the expiratory flow F of a patient does not exist, that is, when the PEFM is not used, the movable plate 12 is placed at a location {circle around (1)} shown in FIG. 1, and the indicator 13 is made to be in close contact with the movable plate 12. When the patient starts to expire, a force of expiratory flow F is applied to the movable plate 12, so that the movable plate 13 starts to move to the right side of the drawing. As a push force obtained by the expiratory flow F increases, a movement distance L is lengthened, and the indicator 13 moves together with the movable plate 12. Simultaneously, the movable plate 12 pulls the spring 11, so that an elastic force is generated and maintained in the spring 11. A patient forcibly expires as in spirometry, which corresponds to a standardized respiratory function examining method of allowing the patient to apply a mouth to the expiration tube 10 and expire as rapidly as possible and as much as possible, after maximally inhaling air. If the value of the push force, generated by the expiratory flow F during the forcible expiration of the patient, starts to decrease from a peak value, the movable  plate 12 starts to move to the left side of the drawing due to the elastic force of the spring 11, and consequently returns to {circle around (P)} its original location {circle around (1)}. However, since the indicator 13, having moved {circle around (m)} while being in close contact with the movable plate 12, is not connected to the movable plate 12, the indicator 13 moves by the right maximum movement distance L of the movable plate 12 and then remains at a location {circle around (2)}, so that peak expiratory flow is measured by visually measuring the maximum movement distance L.
The above-described peak expiratory flow meter is widely utilized medical instrument, because it allows an asthma patient to conveniently carry and occasionally measure peak expiratory flow if necessary. For the prior arts related to the peak expiratory flow meter, there are patents disclosed in UK Pat. No. 1463814 and U.S. Pat. No. 5,224,487.
The peak expiratory flow meters disclosed in the above prior arts are portable and convenient to use, but they can measure only a peak expiratory flow value in a spirometry process executed while a patient forcibly expires. However, typically, the evaluation of the respiratory function of a chronic respiratory disease patient, such as an asthma patient, is possible only when an expiratory flow signal is continuously measured during  the forcible expiration and then important index values, such as Forced Vital Capacity (FVC) and Forced Expiratory Volume at 1 sec (FEV1.0), are obtained together from the measured signal waveform (R. E. Kanner, and A. H. Morris, “Forced expiratory spirogram”, In: Clinical pulmonary function testing, pp. I-7˜10, Intermountain Thoracic Society, Salt Lake City, 1975). However, the above-described conventional peak expiratory flow meters are limited in the evaluation of the respiratory function because they provide only a peak expiratory flow value, which is the maximum value of the expiratory flow.
Further, when FVC, FEV1.0 and PEF, which are important and essential indexes to evaluate the respiratory function, are required to be simultaneously obtained, an expensive spirometer must be used, which has a construction completely different from that of the PEFM in an entire operating structure including the principles of measurement of an expiratory flow signal. Therefore, when an asthma patient desires to currently evaluate the respiratory function thereof, there is a problem in that he or she must measure only a PEF value using the PEFM, or utilize a separate spirometer.